Industrial robots are in widespread use for automated welding. The most prominent advantages of automated welding are precision and productivity. Robot welding improves weld repeatability. Once programmed correctly, robots will give precisely the same welds every time on workpieces of the same dimensions and specifications.
Automating the torch motions decreases the error potential which means decreased scrap and rework. With robot welding you can also get an increased output. Not only does a robot work faster, the fact that a fully equipped and optimized robot cell can run for 24 hours a day, 365 days a year without breaks makes it more efficient than a manual weld cell.
Another benefit of automated welding is the reduced labor costs. Robotic welding also reduces risk by moving the human welder/operator away from hazardous fumes and molten metal close to the welding arc.
The function of the welding electrodes is to conduct the current and to withstand the high pressures in order to maintain a uniform contact area and to ensure the continued proper relationship between selected current and pressure. Uniform contacting areas should therefore be maintained.
Good weld quality is essential and depends, to a considerable degree, upon uniformity of the electrode contact surface. This surface tends to be deformed (mushroomed) with each weld. Primary causes for mushrooming are too soft electrode material, too high welding pressure, too small electrode contact surface, and most importantly, too high welding current. These conditions cause excessive heat build-up and softening of electrode tips. Welding of today's coated materials also tends to contaminate the face of the electrodes.
As the electrode deforms (the tip flattens), the weld control is called upon to “step” up the welding current in order to compensate for “mushroomed” weld tips. Eventually, the production line will have to be shut down in order to replace the electrodes or to manually go in and hand dress the electrodes. This process will improve the weld cycle but in either case, the line is stopped and time is lost. Furthermore the deformed electrodes have caused unnecessary high consumption of energy and electrode material. Bad welds are caused by bad part fit-up, part quality control, weld face control and weld tip position.
In automatic tip dressing, a tip dresser is mounted on the line where it can be accessed by the welding robot. The robot is programmed to dress the electrodes at regular time intervals. The dressing can be done after each working cycle, after every second cycle, and so on. It depends upon how many spot-welds are done in each cycle. Maintaining proper electrode geometry minimizes production downtime and utility costs and increases weld efficiency.
Weld current steppers such as those described in U.S. Pat. Nos. 4,104,724; 4,885,451; 5,083,003; 5,386,096 and 5,449,877 provide an inadequate solution to the weld tip problem. A weld current stepper is a feature of the welding control wherein the welding current is increased (or, in special cases decreased) to compensate for welding electrode wear and deterioration. One way to implement a weld current stepper is to have the control keep track of the number of welds made and increase (or in some special cases, decrease) the welding current according to the number of welds made. Another method of implementing a weld current stepper is to use electrical measurements to identify events during the welding process and increase or decrease the welding current in response to these events. These methods of counting welds have been found to be inadequate to compensate for the variations in both the current and force required for an optimum weld due to the increasing contact area of the electrodes on the surface of the part and the wearing down of the electrode.
Controls have been placed on weld stoppers to simplify the system, however, these welding heat steppers require inputting the total number of welds to be made before dressing the electrodes and the heat percent increase over the original setting to be reached during the last weld. As such, no subjective assessment of the actual working robot weld tip is made.
U.S. Pat. No. 6,639,181 teaches an apparatus and method for assessing electrode tip wear. Tip replacement is determined by a replacement index average assigned based on welding voltage and current measurements. A comparison is made between the assigned value and a reference value.
WO 2000/071291 offers a different solution by providing a method for determining the resistance spot welding system condition having a servomotor-actuated welding machine to measure and control welding electrode force and position allow the welding controller to gain information that is useful to control the resistance spot welding process.
It is desirable to provide an accurate, real time subjective assessment of the weld tip during welding operation to ensure precision and productivity.